Communication devices are known to include data transmitters and receivers. Data transmitters typically operate by continuously modulating a characteristic (e.g., frequency, phase, or amplitude) of a communication medium. It is common in radio frequency (RF) based communication systems to utilize a frequency modulation (FM) technique to continuously modulate the frequency of a carrier signal. In such a scheme, deviation of the carrier signal about its center frequency is proportional to characteristics of an information signal, which information signal is representative of information intended for transmission. For example, the transitions between levels in a digital signal--e.g., ones and zeroes--can be used to cause the frequency of a carrier signal to variously shift above and below the center frequency of the carrier signal.
In practice, FM data transmitters often utilize phase locked loops (PLL's) to generate high-frequency carrier signals synchronized to a substantially lower frequency input reference signal. One problem with PLL's, inherent to their operation, is that they will follow slow variations (i.e., lower frequencies) of the reference signal, but will not follow fast variations (i.e., higher frequencies), such as noise jitter, in the reference signal. Stated another way, lower frequencies are filtered out of the carrier signal by the PLL, whereas the higher frequencies are not filtered out, thus modulating the carrier signal with undesired noise jitter. For this reason, it is important to use a highly stable reference signal as input to a PLL, since it is undesirable that a reference signal decrease the reliability of the data transmitter by causing modulation of the carrier signal.
Commensurate with the aforementioned filtering characteristic of PLL's, information signals used to modulate a PLL are subject to the same constraints as the reference signal; the high frequency content of the information signal applied to the data transmitter modulates the carrier signal, while the low frequency content is typically filtered out. While this lack of DC response (i.e., non-filtering of frequencies approaching 0 Hz) can be tolerated in some cases, there are communications systems, such as simulcast systems, that require DC response for proper operation of the data transmitters.
To provide DC response in data transmitters using PLL's, dual-port modulation can be used. FIG. 1 illustrates a dual-port data transmitter (100) in accordance with such prior art modulation techniques. The dual-port data transmitter (100) includes a reference signal source (102), a reference modulator (104), ant a PLL (106). The reference signal source (102) can be a temperature controlled crystal oscillator (TCXO) and the reference modulator (104) can also be a PLL.
In order to modulate a carrier signal (103) with a larger portion of the spectrum of an information signal (110)--i.e., provide DC response--the information signal (110) is simultaneously applied to the reference modulator (104) and a voltage controlled oscillator (VCO) within the PLL (106). In this manner, the high-pass filtering characteristics of the PLL (106) are mitigated since the low frequency content of the information signal (110) is "added" to the reference signal by the reference modulator (104).
While dual-port data transmitters achieve improved DC responses, they suffer from a variety of drawbacks. From a performance standpoint, any noise added to the reference signal by the reference modulator is magnified in the PLL by a factor of N/R, as exemplified in FIG. 1. Additionally, such data transmitters are not only subject to the various difficulties associated with PLL's (i.e., temperature drift of the VCO, thermal variation of the modulation sensitivity of the VCO, phase equalization of the two modulation ports, etc.), but they also incur the added cost, size, and complexity of the reference modulator Therefore, a need exists for a method and apparatus for modulating a carrier signal in a data transmitter that offers similar performance to existing techniques, and yet overcomes the higher cost and complexity of their implementations.